By comparing the teeth of a score of primates of the past and present, a group of scientists has observed that the size of the fangs of a possible ancestor species of humans was reduced several million years ago. There was also a marked change in the sexual dimorphism (difference between males and females) of the canines, dwarfing those of the males. For the authors of this comparison, the changes towards a less pronounced and aggressive teeth would be related to the appearance of cooperative behaviors and reduction of violence.

About thirty years ago, the magazine Nature published the discovery of a new species, the Australopithecus ramidus. In time, they took away the Australopithecus name and christened it Ardipithecus ramidus. Found in Ethiopia, this hominin lived in the region about 4.4 million years ago. He walked upright and for his discoverers he is a direct ancestor of modern humans. Now they have verified that their teeth were very different from those of other primates of then and later: the canines of their males are smaller and almost equal to those of the females (absence of sexual dimorphism), in particular the superior ones.

One of the discoverers of A. ramidus It was Gen Suwa, a Japanese paleoanthropologist at the University of Tokyo. Now, Suwa and other colleagues, including researchers from the National Center for Research on Human Evolution, in Burgos, have carefully studied their teeth and compared them with those of other hominins of their time, later – such as australopithecines and first sapiens— and other now extinct and current primates, such as chimpanzees, bonobos or gorillas.

“In primates with large canines, the superiors are more important as a weapon and display of force”

Gen Suwa, Japanese paleoanthropologist at the University of Tokyo and co-discoverer of ‘Ardipithecus ramidus’

Suwa’s work, published in the magazine PNAS, shows that already among the A. ramidus By 4.4 million years ago, the difference between male and female tusks had been reduced to a ratio of 1.06 in the upper tusks (that is, they were only 6% larger in the male specimens ) and 1.13 in the lower ones. “It is very likely that the small canines in the males dated back six to seven million years ago,” Suwa notes, but the fossil record prior to A. ramidus it is too poor to go further back. In comparison, the ratio in today’s humans is 1.03 (upper canines) and 1.09 (lower). In chimpanzees, the last lineage from which hominids separated millions of years ago, the upper tusks have a ratio of 1.34 and the lower ones, 1.28, being in total one third larger than those of their female partners.

A denture so powerful and with such a difference in favor of males is only found by gorillas and two species of hominids that predate the australopithecines of more than 10 million years ago, the Nacholapithecus kerioi and the Hispanopithecus laietanus, with 30% larger teeth and a sexual dimorphism of almost 70%. In general, in the most primitive hominids and australopithecines, in addition to greater volume, the tusks are longer, diameter extended at the base, sharp and oriented outwards. Unlike the canines of carnivores, which are designed to grab and tear, those of primates seem made for aggression or defense.

Suwa argues that “in most primates with large canines, the upper ones are more important as a weapon and / or display [de fuerza]”. And this appears related to what they have observed: the reduction is more pronounced in the upper fangs, precisely where sexual dimorphism was greatest before the arrival of the hominins. “So if there was selection for a less aggressive behavior, it is possible that there was selection for a less threatening canine, and this would have affected the superior more than the inferior,” Suwa details.

That is the thesis that the authors of the study maintain: that there is a relationship between the shrinking of canines and changes in the behavior of those groups of hominids that would bring them closer to current humans. The A. ramidus he lived in social groups in which cooperation would pay off more than competition, especially between males and for females.

“We would be before one of the first steps towards what we call human. The reduction of the canines would coincide in time with the appearance of bipedalism, so humanity as such began when we stood up and began to reduce the levels of aggressiveness “

María Martinón-Torres, director of the National Research Center on Human Evolution

To reinforce their thesis they resort to comparative analysis. Among the great apes there is only one species with such a reduced sexual dimorphism in canines, bonobos, with ratios close to 1.20. The use of sexuality among these primates as a reducing mechanism for aggressiveness and conflict is well known. Also primates, but further down the evolutionary tree, are the species of the genus Brachyteles, among which are the woolly spider monkeys of Brazil. As in humans, the difference between male and female fangs is very small. Among these monkeys, a philopatric social organization predominates (communities formed by specimens born in the place) with diverse females and males with a polygynous reproductive strategy (non-exclusive mating between males and females) and where there is “an extreme tolerance between males and little aggression in competition for sexual access to receptive females, ”the study authors write.

The director of CENIEH (National Center for Research on Human Evolution), María Martinón-Torres, highlights how important it is to know the sex of the fossils. “It has many implications because, in addition to helping us understand the variability of a sample, it provides information on the ecology and behavior of that species, particularly on the degree of competition between males.” In the case of this study, “it reveals that the first hominin already had relatively small canines, which allows us to infer that it probably had a less aggressive behavior than other previous primates.” And this points to a series of momentous changes: “We would be facing one of the first steps towards what we call human. The reduction of canines would coincide in time with the appearance of bipedalism, so humanity as such began when we stood up and began to reduce the levels of aggressiveness. Those would be the first hallmarks of humans. “

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